By Darren Hefty
How could a tiny bug cause this much damage? That’s what people used to say to me a few years ago. Who could blame them? After all, we’d never had aphids attacking our soybeans before. Well, it didn’t take long to realize that 5 or 10 bushels of yield could be gone in a short time if soybean aphids weren’t stopped quickly. The questions were: what products to use, how long would they last, and when to pull the trigger.
WHAT PRODUCTS TO USE?
If aphid counts are high: When it comes to stopping aphids fast, the product of choice is Lorsban. There are also combination products like Cobalt, which has Lorsban in it.
If aphid counts are low to moderate: The pyrethroid family of insecticides has been by far and away the most popular aphid solution. Products like Silencer, Declare, Warrior, Baythroid, and Asana provide knockdown and residual. Did I mention you can get a full rate of some of these pyrethroids for $2/Acre or even less? Some of the pyrethroids come with a re-spray policy, which is not to be confused with a guarantee of long-lasting residual control. If the re-spray program is for 30 days, for example, that doesn’t mean go to the lake after you spray and come back to check on the field in 30 days. It simply means if you have to respray within 30 days, they’ll cover part or all of the cost of the spray.
HOW LONG WILL THEY LAST?
We expect to get 10 to 14 days of residual control out of the full rates of the pyrethroid products like Silencer or Declare. We expect to get a little less residual, like 7 to 10 days, out of Lorsban. If you get more rainfall after application, your residual control is likely to be less.
WHEN TO PULL THE TRIGGER?
This is the toughest part, but not because of cost. Here are two examples.
- If you don’t see any bugs at all, but you want to spray for weeds or disease prevention, DON’T SPRAY INSECTICIDE! There are beneficial insects in your fields that will be wiped out by spraying an unnecessary pass of insecticide.
- The definition of an economic threshold is when you have enough insects to financially justify the treatment to offset potential yield loss. If 10 aphids per plant at R2 (full flower) costs you more in terms of yield than your cost of application, you’ve reached the economic threshold. With the changing economics over the last 5 years, soybean prices have doubled while insecticide prices have greatly decreased. The true economic threshold can no longer reasonably be 250 aphids per plant, if it ever was to begin with. We start spraying on our farm at 10 to 20 aphids per plant at R2. As the season progresses, it takes more aphids to justify the cost of the treatment for the potential yield you’re saving. If you have to hire a plane for $8/Acre to fly on the insecticide, you must figure that into the equation, as well.
What if there are other bugs like spider mites? Use Lorsban or Capture for aphid control if there are spider mites in the area. Other insecticides will not kill the mites, and mite populations could explode when you wipe out all the competition and predators. Be careful.
Should I use higher rates for more residual? You may only gain a few days of residual, but for $1.00 or less it’s probably worth it.
Should I rotate modes of action? This is a great idea with everything you do on the farm. Rotate. Rotate. Rotate. Use a pyrethroid like Silencer or Declare for one application and Lorsban (an organophosphate) for the next application or vice versa.
Should I add a fungicide? When insects like aphids are on your plants, they pierce the leaves and suck out the juice. They also leave an open wound for disease to enter the plant. Some insects are also vectors for disease, meaning they actually spread the disease from field to field and plant to plant. On our farm, we almost always see better yield gains and returns on our investments when we add a fungicide like Headline to our insecticide applications. Last year, we saw more than a 7 bushel per acre gain using fungicide plus insecticide on our soybeans!
Soybean aphids can destroy your yields. Err on the side of too early rather than too late when planning your insecticide applications. Keep scouting until the plants begin to turn yellow in the fall. We’ve seen positive yield gains with insecticide applications well into August.